The present invention relates to a discharge processing device which processes a work piece by generating a discharge in a processing gap which is formed between an electrode and the work piece. More particularly, the present invention relates to a discharge process power supply in which a switching element which generates a high frequency electrical power pulse is connected between a direct current power source and a processing gap.
When power from a direct current power is applied to the processing xe2x80x9cgapxe2x80x9d, a small gap formed between the tool electrode of an electric discharge processing device and a conductive work piece, the resistance of the dielectric liquid within the gap declines. When the insulating characteristic of the dielectric liquid is destroyed, a discharge is generated and machining xe2x80x9con-timexe2x80x9d begins. A discharge current flows through the gap during a controlled xe2x80x9con-timexe2x80x9d and as a result, work piece material evaporates or is dissolved. When the on-time is completed, application of power to the gap is stopped so that the insulation characteristics of the dielectric liquid are restored during the xe2x80x9coff-timexe2x80x9d. In general, switching circuits made up of many switching transistors are set between the direct current power source and the gap in order to repeatedly supply high frequency power pulses from the direct current power source. The on-time/off-time switching operation of the switching transistor is controlled by gate pulse signals which have a controlled on-time and a controlled off-time. It is well known that minimizing the RL in the power supply circuit which supplies power from the direct current power source to the gap via the switching circuit as much as possible in order to decrease the rise time of the current which flow through the gap contributes to increasing the processing speed. For example, under conditions including a 280 V direct current power source output voltage, 0.02 Q power supply circuit resistance and 0.4 pH inductance, the current which flows through the gap rises as much as 3000 A to 1000 A within 5 xcexcsec and reaches a saturation current of 14,000 A. As the switching circuit must withstand a saturation current of 14,000 A, the circuit requires 280 50 A transistors. Nevertheless, the maximum value of the discharge processing current pulse which is used in a wire cut electric discharge machine is approximately 1000 A to 2000 A so that the switching circuit of such a wire cut electric discharge machine usually comprises only 20 or 30 50 A transistors. As a result, when the on-time for the gate pulse signals exceeds the indicated value due to static and/or other factors, a large number of transistors may be damaged by excess current.
It is an object of the present invention to provide a power supply for electric discharge machining having a contact circuit which reliably protects the switching transistors, even if the on-time of the gate signal pulses exceeds the indicated value.
Other objects of the present invention will be related in the explanation which follows and/or will become clear to a person skilled in the art by practice of the invention.
In order to attain these objectives, a power supply for an electric discharge machine according to the invention is provided which intermittently supplies electric power for electric discharge processing of a machining gap which is formed between an electrode and a work piece, comprises:
a direct current power source;
a switching device connected between the direct current power source and the machining gap;
a gate pulse generator circuit for generating gate pulse signals to energize the switching device;
a clock pulse generator for generating clock pulses;
a counter for counting clock pulses while receiving gate pulse signals and for generating a predetermined level output when the pulse count reaches a predetermined level; and
a circuit for maintaining the counter at a predetermined level while gate pulse signals are being received and preventing the gate pulse signals from being supplied to the switching element.